Each Saturday, tourists can climb aboard for an express trip to Castel Gandolfo, a lavish estate Pope Francis has never used but wanted to share with the public in a gesture which will also boost the Church's coffers.
From San Pietro, to Trastevere and Ostiense, the train chugs through the Italian capital and its green suburbs, down past the Roman ruins on the Via Appia, before climbing through the Alban hills to the picturesque Lake Albano.
Some 25 kilometres (15 miles) from Rome, it pulls to a stop at the castle town, a former summer favourite with popes hoping to escape the heat of the capital, and still frequented occasionally by Francis's predecessor, Benedict XVI.
The villa and gardens, owned by the Holy See since 1596, expanded over the centuries to include other properties and now sprawl over 55 hectares (135 acres).
Inside the grounds, there are views down to the lake or glimpses of the sea beyond gardens decorated with sculptures. There are also orchards of apricot, peach and olive trees, and greenhouses of ornamental flowers.
The estate's gardens first opened to the public in 2014, with tours organized for groups and by reservation only.
But workaholic Francis, who does not take vacations and has only been to the summer palace twice, urged his Museum's director to go further, opening up the entire estate and setting up a new museum.
'Closed for centuries'
Visitors to the villa can stand where popes down the centuries stood to bless summer Sunday crowds - the very window from which Benedict XVI said his last goodbyes before retiring from the papacy.
As well as a papal portrait gallery, visitors can pore over embellished vestments worn by the holders of Saint Peter's Chair down the ages, including elegant papal slippers of the type shunned by down-to-earth Francis.
For €40, tourists get the full package: they can skip the queue in the morning to see the Vatican Museums and Sistine Chapel in Rome, before walking through the Vatican's gardens to the train station.
Once at Castel Gandolfo, they hop on a special white tourist train which takes them around the papal villa, including past the pope's organic farm, which houses cows, free-range hens, cockerels and pontifical bees.
From the train's windows they get a glimpse at the makings of the papal menu: freshly laid eggs, milk, vegetables, fruit, honey and fat rounds of caciotta cheese from the farm often end up on the pope's table back in the Vatican.
Visitors can then wander around the town of Castel Gandolfo, a mediaeval borough listed among Italy's most beautiful, before getting the train back to the Vatican in the afternoon.
There is also a €16 option for those who want to skip the Vatican Museums and just do Castel Gandolfo.
Frugal Francis, who has shunned the Holy See's papal apartments for life in a Vatican hotel, has said he has no time to visit the estate - a decision, however, which has seen local shopkeepers hit by a drop in tourism.
The Argentine hopes that by launching the new link with the summer palace it will revive the struggling businesses.
"Seeing as he's unable to spend time at Castel Gandolfo because of his numerous engagements, Francis wanted to make a generous gesture so that we can all visit a place which has been closed for so many centuries," Osvaldo Gianoli, director of the Castel Gandolfo villas, told AFP.